Power supply device and lighting equipment

ABSTRACT

In a power supply device according to one embodiment, a reference signal (Vref 1 ), which changes from a value corresponding to a maximum current in a full lighting state to that corresponding to a minimum current in case of a deepest dimming depth, and a reference signal (Vref 2 ), which changes from a value corresponding to a load voltage at the time of a maximum current in a full lighting state to that corresponding to a minimum current in case of the deepest dimming depth, are prepared in accordance with dimming depths of a dimming signal. In a shallow dimming depth region close to a full lighting state, the reference signal (Vref 1 ) is selected to apply constant-current control to light-emitting diodes in a current control mode. In a deep dimming depth region, the reference signal (Vref 2 ) is selected to apply constant-voltage control to the diodes in a voltage control mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of U.S. application Ser. No.12/873,744, filed on Sep. 1, 2010, which is now U.S. Pat. No. 8,354,804,U.S. application Ser. No. 12/873,744 is a Continuation Application ofPCT Application No. PCT/JP2009/055871, filed Mar. 24, 2009, which claimsthe benefit of priority from Japanese Patent Application No.2008-076835, filed Mar. 24, 2008. The entirety of all of theabove-listed applications are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a power supply deviceand a lighting equipment, which have a dimming function for driving asemiconductor light-emitting element such as a light-emitting diode.

BACKGROUND

Recently, power supply devices which switch DC power using switchingelements are popularly used for driving semiconductor light-emittingelements such as light-emitting diodes. These power supply devicesinclude a dimming function for controlling the amount of light emittedby a light-emitting diode in accordance with an externally input dimmingsignal.

Conventionally, as disclosed in, e.g., JP-A 2003-157986 (KOKAI), a powersupply circuit includes both a voltage dimming Circuit that controls anapplied voltage to a light-emitting diode, and a duty dimming circuitthat switches on and off the applied voltage to the light-emittingdiode. A dimming control signal controls switching between the voltagedimming circuit and duty dimming circuit.

In the power supply circuit disclosed in JP-A 2003-157986 (KOKAI),dimming control is carried out based on a pulse width. Therefore, thelight output from the light-emitting diode may flicker. Also, a switchelement connected in series or parallel to the light-emitting diode isrequired in addition to a current-limiting element for output currentcontrol based on the pulse width control. Therefore, the number ofcomponents of the power supply circuit is increased, and the efficiencyof the power supply circuit drops.

However, since a light-emitting diode exhibits nearly constant voltagecharacteristics, a component or circuit having a current-limitingelement is required to stably light on the light-emitting diode. Ingeneral, when the light-emitting diode is controlled by a power supplydevice using a switching element, current control is adopted. Thiscurrent control is an important control element in design of a lightingdevice since the temperature of the light-emitting diode is decided by acurrent value to be supplied to the light-emitting diode, and itinfluences a service life of the element.

Dimming the light-emitting diode is relatively easily attained comparedto a conventional electric discharge lamp lighting device. This is basedon the fact that the light-emitting diode as a load has electricallystable characteristics, and suffers less variations of itscharacteristics against an external factor such as a temperature.However, when constant current control is adopted in an application thatrequires deep dimming, the light-emitting diode can be stably lighted ina region with a large full lighting current, but a current detectionsignal or a current reference value required to control this currentdetection signal becomes a very low signal in a deep dimming region.Therefore, a detection circuit or comparator, which detects a current,is required to have high precision, and it becomes difficult to attainstable operation due to high susceptibility to noise. Hence, a signalvoltage required for control may be increased. However, in general, acurrent detection signal is detected by a resistor inserted in series tothe light-emitting diode, and consumption power and heat generationamounts by this resistor increase in a region where a current flowingthrough the light-emitting diode is large, thus disturbing developmentof the power supply device.

As a proposal which solves these problems, a method ofconstant-controlling an output voltage has been proposed. For example,an ON voltage of a light-emitting diode is higher than that of a generalsilicon diode. For example, in a GaN diode represented by a bluelight-emitting diode, current begins to flow from about 2.5 V, and afull lighting state requires a voltage as low as about 3.5 to 4.5 V.Even for deep dimming, relatively stable dimming can be attained withoutbeing influenced by the performance of the light-emitting diode ornoise. However, since a forward voltage of the light-emitting diode hasnegative temperature characteristics, the light-emitting diode isself-heated by a flowing current. As a result, since the forward voltagelowers and a current further increases, a heat generation amount maybecome larger, thus causing thermal runaway. Also, light-emitting diodeshave large variations, and output currents vary due to individualdifferences of light-emitting diodes even when the output of a lightingdevice is adjusted. This problem is posed similarly in a dimmed state inaddition to a full lighting state. As a result, the currents flowingthrough light-emitting diodes vary, resulting in variations of lightoutputs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing a lighting equipmenthaving a power supply device according to the first embodiment;

FIG. 2 is a schematic sectional view showing the internal structure ofthe lighting equipment shown in FIG. 1;

FIG. 3 is a schematic circuit diagram showing an electrical circuit ofthe power supply device shown in FIG. 1;

FIG. 4 is a graph showing reference signals Vref1 and Vref2 generatedfrom a reference signal output unit shown in FIG. 3;

FIG. 5 is a graph showing the relationship between V-I characteristicsof each light-emitting diode shown in FIG. 3 and load characteristicsspecified by a dimming signal;

FIG. 6 is a graph showing the V-I characteristics of each light-emittingdiode shown in FIG. 3;

FIG. 7 is a schematic circuit diagram showing a power supply circuit ofa power supply device according to the second embodiment;

FIG. 8 is a graph showing the relationship between V-I characteristicsof each light-emitting diode shown in FIG. 7 and load characteristicsspecified by a dimming signal;

FIG. 9A is a circuit block diagram showing a reference signal outputunit in a power supply circuit according to a modification of the powersupply device shown in FIG. 7;

FIG. 9B is a graph for explaining reference signals output from thereference signal output unit shown in FIG. 9A;

FIG. 10A is a graph for explaining a reference signal generated by thereference signal output unit shown in FIG. 9A;

FIG. 10B is a graph for explaining another reference signal generated bythe reference signal output unit shown in FIG. 9A;

FIG. 11A is a circuit block diagram showing a reference signal outputunit in a power supply circuit according to another modification of thepower supply device shown in FIG. 7;

FIG. 11B is a graph for explaining reference signals output from thereference signal output unit shown in FIG. 11A;

FIG. 12 is a schematic circuit diagram showing a power supply circuit ofa power supply device according to another embodiment;

FIG. 13 is a graph showing load characteristics of the power supplydevice shown in FIG. 12;

FIG. 14 is a graph showing V-I characteristics of each light-emittingdiode shown in FIG. 12;

FIG. 15A is a graph for explaining a variation range of eachlight-emitting diode shown in FIG. 12; and

FIG. 15B is a graph for explaining a variation range of eachlight-emitting diode shown in FIG. 12.

DETAILED DESCRIPTION

Embodiments of a lighting equipment including a power supply devicewhich can implement dimming control will be described in detailhereinafter with reference to the drawings.

In general, according to one embodiment, a power supply device comprisesa semiconductor light-emitting element, a current control circuit, avoltage control circuit, and a dimming controller. The current controlcircuit controls a current supplied to the semiconductor light-emittingelement, and the voltage control circuit controls a voltage applied tothe semiconductor light-emitting element. The dimming controllerreceives a dimming signal having a certain dimming depth, sets a currentcontrol mode and/or a voltage control mode according to the dimmingdepth. The dimming controller controls the current control circuit todrive the semiconductor light-emitting element at a target currentspecified depending on the dimming depth when the current control modeis set, and also controls the voltage control circuit to drive thesemiconductor light-emitting element at a target voltage specifieddepending on the dimming depth when the voltage control mode is set.

(First Embodiment)

FIGS. 1 and 2 show a lighting equipment which incorporates a powersupply device according to an embodiment. Referring to FIGS. 1 and 2,reference numeral 1 denotes an equipment main body. This equipment mainbody 1 is prepared by die-casting aluminum, and is formed into a nearlycylindrical shape having openings at two ends. The interior of thisequipment main body 1 is partitioned into three spaces in a verticaldirection by partition members 1 a and 1 b. In a lower space between alower opening and the partition member 1 a, a light source unit 2 isarranged. This light source unit 2 includes a plurality of LEDs 2 a assemiconductor light-emitting elements and a reflector 2 b for reflectinglight rays from the LEDs 2 a. The plurality of LEDs 2 a are mounted inthe lower space, and are allocated at equal intervals along acircumferential direction of a disk-shaped circuit board 2 c arranged onthe lower surface of the partition member 1 a.

A hollow space between the partition members 1 a and 1 b of theequipment main body 1 is assigned to a power supply chamber 3. In thispower supply chamber 3, a circuit board 3 a is arranged on an upperportion of the partition member 1 a. On this circuit board 3 a,electronic components which configure a power supply device required todrive the plurality of LEDs 2 a are arranged. This DC power supplydevice and the plurality of LEDs 2 a are connected via lead wires 4.

A space between the partition member 1 b and an upper opening of theequipment main body 1 is defined as a power supply terminal chamber 5.In this power supply terminal chamber 5, a power supply terminal block 6is arranged on the partition member or plate 1 b. This power supplyterminal block 6 is a terminal block required to supply an AC power of acommercial power supply to the power supply device in the power supplychamber 3, and has outlets 6 b as power supply terminals for a powersupply cable, outlets 6 c used as terminal portions for a feeder cable,a release button 6 d used to release a power supply line and feederline, and the like on two surfaces of a box 6 a which is made up of anelectrically insulating synthetic resin.

FIG. 3 is a circuit diagram of the power supply device according to theembodiment, which is incorporated in the power supply chamber 3 of thelighting equipment with the above arrangement.

Referring to FIG. 3, reference numeral 11 denotes an AC power supply asa commercial power supply outside the lighting equipment. This AC powersupply 11 is connected to power supply terminals 6 b of the lightingequipment shown in FIG. 2 via a lighting switch (not shown) outside thelighting equipment, and a full-wave rectifying circuit 12 is connectedto the power supply terminals 6 b. The full-wave rectifying circuit 12outputs a rectified voltage obtained by full-wave rectifying an AC powerfrom the AC power supply 11 upon an ON operation of the lighting switch.Between output terminals of the full-wave rectifying circuit 12, asmoothing capacitor 13 which smoothes a ripple current is connected inparallel. The full-wave rectifying circuit 12 and capacitor 13 form a DCpower supply circuit, which is connected to a primary winding 14 a of aswitching transformer 14 as a flyback transformer.

To the primary winding 14 a of the switching transformer 14 as a flybacktransformer, a field effect transistor (FET) 15 as a switching elementis connected in series. Between the two terminals of the smoothingcapacitor 13, a series circuit of the primary winding 14 a of theswitching transformer 14 and FET 15 is connected. The switchingtransformer 14 has a secondary winding 14 b which is magneticallycoupled to the primary winding 14 a.

To the secondary winding 14 h of the switching transformer 14, arectifying/smoothing circuit 18, which rectifies and smoothes a voltagegenerated at the secondary winding 14 b, is connected. Therectifying/smoothing circuit 18 includes a diode 16 which is connectedin series to the secondary winding 14 b, and a smoothing capacitor 17which is connected in parallel to the secondary winding 14 b. Thisrectifying/smoothing circuit 18 forms a DC lighting circuit whichgenerates a DC output required to light on light-emitting diodestogether with the FET 15 and switching transformer 14.

In this DC lighting circuit, when the FET 15 is turned on and off inresponse to pulse signals having a certain ON duty ratio output from acontrol circuit 30, a DC voltage from the full-wave rectifying circuit12 is converted into a rectangular wave voltage, which is applied to theprimary winding 14 a of the switching transformer 14. When thisrectangular wave voltage appears at the primary winding 14 a of theswitching transformer 14, a boosted AC voltage is generated from thesecondary winding 14 b of the switching transformer 14. This AC voltageis rectified by the diode 16 in the rectifying/smoothing circuit 18, therectified voltage is smoothed by the smoothing capacitor 17, and thesmoothed voltage is output from the smoothing capacitor 17 as a DCoutput.

Between the two terminals of the smoothing capacitor 17 in therectifying/smoothing circuit 18, a plurality of series-connectedlight-emitting diodes 19 to 21, for example, three series-connectedlight-emitting diodes 19 to 21 as semiconductor light-emitting elementsare connected as loads. The light-emitting diodes 19 to 21 correspond tothe LEDs 2 a as light sources shown in FIG. 1. The series-connectedlight-emitting diodes 19 to 21 are dimmed and lighted on when they aresupplied with a DC current according to a certain DC voltage output fromthe rectifying/smoothing circuit 18. That is, when the FET 15 is turnedon and off in response to switching pulses having a high ON duty ratio,an AC voltage, which is boosted to a relatively high level, appears fromthe secondary winding 14 b of the switching transformer 14, a relativelyhigh DC voltage is applied from the rectifying/smoothing circuit 18 tothe light-emitting diodes 19 to 21, and a constant current is suppliedto the light-emitting diodes 19 to 21 to light them on at a certainluminance level. When the FET 15 is turned on and off in response toswitching pulses having a low ON duty ratio, an AC voltage, which isboosted to a relatively low level, appears from the secondary winding 14b of the switching transformer 14, and a relatively high DC voltage isapplied from the rectifying/smoothing circuit to the light-emittingdiodes 19 to 21, thus dimming and lighting on the light-emitting diodes19 to 21.

To a series circuit of the light-emitting diodes 19 to 21, a currentdetection circuit 22 is connected in series. This current detectioncircuit 22 includes a resistor 221 as an impedance element. In order todetect a current flowing through the light-emitting diodes 19 to 21, aseries circuit of the light-emitting diodes 19 to 21 and the resistor221 is connected in parallel to the smoothing capacitor 17, and acurrent detection value I is output from a node between the seriescircuit of the light-emitting diodes 19 to 21 and the resistor 221 as adetection signal. To the anode side of the series circuit of thelight-emitting diodes 19 to 21, a load voltage detection circuit 23 isconnected, and the anode side is grounded via this load voltagedetection circuit 23. This load voltage detection circuit 23 includes aseries circuit of resistors 231 and 232 as impedance elements, detects aload voltage applied to the series circuit of the light-emitting diodes19 to 21, i.e., a voltage on the anode side of the light-emitting diode19, and outputs this load voltage V as a detection signal.

To the current detection circuit 22 and load voltage detection circuit23, a dimming controller 33 as a dimming control circuit is connected.The dimming controller 33 includes comparator 34 and 35, and a referencesignal output unit 36. An inverting input terminal of the comparator 34is connected to the node between the series circuit of thelight-emitting diodes 19 to 21 and the current detection circuit 22, anda non-inverting input terminal thereof is connected to the referencesignal output unit 36. The comparator 34 compares a detection signal(current detection value I) of the current detection circuit 22 and areference signal Vref1 of the reference signal output unit 36, andoutputs a comparison result. An inverting input terminal of thecomparator 35 is connected to the node between the resistors 231 and232, and a non-inverting input terminal thereof is connected to thereference signal output unit 36. The comparator 35 compares a detectionsignal (load voltage V) of the load voltage detection circuit 23 and areference signal Vref2 of the reference signal output unit 36, andoutputs a comparison result. These comparators 34 and 35 arerespectively connected to diodes 37 and 38, and are connected to thecontrol circuit 30 via a common node C on the cathode side of the diodes37 and 38. The diodes 37 and 38 form an OR circuit. At the node Cbetween these diodes 37 and 38, a larger one of the reference signalsVref1 and Vref2 appears, and is output to the control circuit 30 as acontrol signal Vcont.

The control circuit 30 controls the FET 15 to be turned on and off by anoperation according to the control signal Vcont so as to switching-drivethe switching transformer 14, thereby controlling an output to besupplied to the light-emitting diodes 19 to 21. The control circuit 30is configured by a switching pulse generation circuit whose ON dutyratio is specified according to the level of the control signal Vcont.For example, the control circuit 30 includes a memory which is referredto by the control signal Vcont, an arithmetic circuit which generatespulse signals at an ON duty ratio stored in this memory, and anamplifier which amplifies pulses output from this arithmetic circuit.

The reference signal output unit 36 is connected to a dimming operationmember 31 arranged on, e.g., a wall surface outside the lightingequipment via a terminal 6 c (corresponding to the outlet 6 c), andreceives a dimming signal k from the dimming operation member 31. Thedimming signal k from the dimming operation member 31 has, for example,levels of dimming depths k1 to k7. In this case, the dimming depth k1 isshallowest. In other words, the light-emitting diodes are dimmed to bebrightest at the dimming depth k1. The light-emitting diodes are dimmedto be deeper toward the dimming depth k7, i.e., to be darker accordingto the value of the dimming depth k. At the dimming depth k7, thelight-emitting diodes are dimmed to be darkest.

The reference signal output unit 36 outputs reference signals Vref1 andVref2, which are selected from lines of the reference signals Vref1 andVref2, as shown in FIG. 4, in accordance with the input dimming signalk. More specifically, the reference signal output unit 36 includes amemory which is referred to by the level of the dimming signal k, and aprocessor which searches the memory using the level of the dimmingsignal k, and outputs Vref1 and the reference signal Vref2 according tothe dimming signal k. The line of the reference signal Vref1 representsa change from a reference signal value corresponding to a maximumcurrent in a full lighting state when the dimming depth is shallowest(dimming depth k1) to that corresponding to a minimum current when thedimming depth is deepest (in case of the dimming depth k7). The line ofthe reference signal Vref2 represents a change from a reference signalvalue corresponding to a load voltage at the time of a maximum currentin a full lighting state when the dimming depth is shallowest (dimmingdepth k1) to that corresponding to a load voltage at the time of aminimum current when the dimming depth is deepest (in case of thedimming depth k7).

Even when the light-emitting diodes 19 to 21 transit to a light-offstate in which nearly no current flows, a certain load voltage can beapplied to the light-emitting diodes 19 to 21, and the reference signalvalue of the reference signal Vref2 is not set to be zero but a certainvalue. FIG. 6 is a graph showing the V-I characteristics of eachlight-emitting diode shown in FIG. 3. The light-emitting diodes 19 to 21have a current control range B and voltage control range C. as shown inFIG. 6, and these ranges have a relationship of light-emitting diode V-Icharacteristics A between them. As can be seen from the light-emittingdiode V-I characteristics A, even when a current value If lowers tonearly zero in the current control range B, a certain voltage Vf can beapplied in the voltage control range C, and this means that even whenthe reference signal Vrefl reaches nearly zero, the reference signalVref2 does not become zero.

FIG. 5 shows the relationship between the dimming depths K1 to K7 to beset and the light-emitting diode V-I characteristics A. The power supplydevice is operated based on different load characteristics according tothe dimming depths k1, k2, . . . , k7 of the dimming signal k shown inFIG. 5. That is, at the dimming depths k1 to k4, the power supply deviceis controlled in a current control mode so as to give loadcharacteristics at operation points a1 to a4 corresponding tointersections between the dimming depths k1 to k4 and the V-Icharacteristics A to the power supply device. In the current controlmode, even when the voltage Vf is changed within a range of theoperation points a1 to a4, the current If is controlled to be a certainconstant target current. An operation point a5 corresponding to anintersection between the dimming depth k5 and the V-I characteristics Ais a critical point, and the current and voltage of the power supplydevice are controlled based on the operation point a5. At the dimmingdepths k6 and k7, the power supply device is controlled in a voltagecontrol mode so as to give load characteristics at operation points a6and a7 corresponding to intersections between the dimming depths k6 andk7 and the V-I characteristics A to the power supply device. In thevoltage control mode, even when the current If is changed within a rangeof the operation points a6 and a7, the voltage Vf is controlled to acertain target predetermined voltage.

The reference signals Vref1 and Vref2 are set as follows in response tothe settings of dimming depth K1 to K7, and the control circuit 30controls the rectifying/smoothing circuit 18 to set a current or voltageoutput to be supplied to the light-emitting diodes 19 to 21 to be aconstant target current or target voltage by controlling the ON/OFFoperations of the switching transistor 15 based on the outputs from thecomparators 34 and 35 according to the set reference signals Vref1 andVref2. That is, in the current control mode, the control circuit 30controls the rectifying/smoothing circuit 18 so that a current suppliedto the light-emitting diodes 19 to 21 reaches a predetermined targetcurrent value. In the voltage control mode, the control circuit 30controls the rectifying/smoothing circuit 18 so that a voltage suppliedto the light-emitting diodes 19 to 21 reaches a predetermined targetvoltage value.

More specifically, when the dimming operation member 31 outputs thedimming signal k for a full lighting state, i.e., with the dimming depthk1, load characteristics corresponding to the dimming depth k1 shown inFIG. 5 are specified according to this dimming signal k, and the powersupply circuit is operated based on the load characteristics. In thiscase, the load characteristics corresponding to the dimming depth k1 arespecified to operate at the operation point a1 of the light-emittingdiodes 19 to 21 corresponding to the intersection between the dimmingdepth k1 and the V-I characteristics A of the light-emitting diodes 19to 21. The reference signal output unit 36 sets a value Vra1 as thereference signal Vref1 and a value Vrb1 as the reference signal Vref2according to the dimming depth k1, as shown in FIG. 4, and outputs thereference signal Vref1 having the value Vra1 and the reference signalVref2 having the value Vrb1. Therefore, the comparator 34 outputs acomparison result between the detection signal (current detection valueI) of the current detection circuit 22 and the reference signal Vra1 ofthe reference signal output unit 36. Also, the comparator 35 outputs acomparison result between the detection signal (load voltage V) of theload voltage detection circuit 23 and the reference signal Vrb1 of thereference signal output unit 36. Since the reference signals satisfy arelation Vra1>Vrb1, the output from the comparator 34 is preferentiallysupplied to the control circuit 30 at the node between the diodes 37 and38. As a result, the ON/OFF operations of the switching transistor 15are controlled based on the detection signal (current detection value ofthe current detection circuit 22 so that a current flowing through thelight-emitting diodes 19 to 21 becomes constant.

Next, upon operation of the dimming operation member 31, when thedimming signal k with, e.g., the dimming depth k5 is output, loadcharacteristics corresponding to the dimming depth k5 shown in FIG. 5are specified. The load characteristics corresponding to the dimmingdepth k5 are specified to operate at the operation point a5 of thelight-emitting diodes 19 to 21 corresponding to the intersection withthe V-I characteristics A of the light-emitting diodes 19 to 21. Thereference signal output unit 36 sets a value Vra5 as the referencesignal Vref1 and a value Vrb5 as the reference signal Vref2 according tothe setting of the dimming depth k5, as shown in FIG. 4, and outputs thereference signal Vref1 having the value Vra5 and the reference signalVref2 having the value Vrb5. Therefore, the comparator 34 outputs acomparison result between the detection signal (current detection valueI) of the current detection circuit 22 and the reference signal Vra5 ofthe reference signal output unit 36. Also, the comparator 35 outputs acomparison result between the detection signal (load voltage V) of theload voltage detection circuit 23 and the reference signal Vrb5 of thereference signal output unit 36. Since the reference signals satisfy arelation Vra5=Vrb5, the outputs from the comparators 34 and 35 becomeequal to each other at the node between the diodes 37 and 38. As aresult, the control circuit 30 controls the ON/OFF operations of theswitching transistor 15 based on both the detection signal (currentdetection value I) of the current detection circuit 22 and the detectionsignal (load voltage V) of the load voltage detection circuit 23.

Next, when the dimming operation member 31 outputs the dimming signal kwith, e.g., the dimming depth k6, load characteristics corresponding tothe dimming depth k6 shown in FIG. 5 are specified. The loadcharacteristics corresponding to the dimming depth k6 are specified tooperate at the operation point a6 of the light-emitting diodes 19 to 21corresponding to the intersection with the V-I characteristics A of thelight-emitting diodes 19 to 21. The reference signal output unit 36 setsa value Vra6 as the reference signal Vref1 and a value Vrb6 as thereference signal Vref2 according to the setting of the dimming depth k6,as shown in FIG. 4, and outputs the reference signal Vref1 having thevalue Vra6 and the reference signal Vref2 having the value Vrb6.Therefore, the comparator 34 outputs a comparison result between thedetection signal (current detection value I) of the current detectioncircuit 22 and the reference signal Vra6 of the reference signal outputunit 36. Also, the comparator 35 outputs a comparison result between thedetection signal (load voltage V) of the load voltage detection circuit23 and the reference signal Vrb6 of the reference signal output unit 36.Since the reference signals satisfy a relation Vra1>Vrb1, the outputfrom the comparator 35 is preferentially supplied to the control circuit30 at the node between the diodes 37 and 38. As a result, the ON/OFFoperations of the switching transistor 15 are controlled based on thedetection signal (load voltage V) of the load voltage detection circuit23 so that a load voltage of the light-emitting diodes 19 to 21 becomesconstant.

As described above, the reference signal Vref1, which changes from asignal value corresponding to a maximum current in a full lighting stateto that corresponding to a minimum current in case of the deepestdimming depth, and the reference signal Vref2, which changes from asignal value corresponding to a load voltage at the time of a maximumcurrent in a full lighting state to that corresponding to a minimumcurrent in case of the deepest dimming depth, are prepared in accordancewith the dimming depths k1, k2, . . . , k7 of the dimming signal k. In ashallow dimming depth region close to a full lighting state, thereference signal Vref1 is selected to apply constant-current control tothe light-emitting diodes 19 to 21 in the current control mode. In adeep dimming depth region, the reference signal Vref2 is selected toapply constant-voltage control to the light-emitting diodes 19 to 21 inthe voltage control mode. According to such control, theconstant-current control and constant-voltage control can be smoothlyswitched according to the dimming depths k1, k2, . . . , k7 of thedimming signal k. As a result, dimming control over a broad range from ashallow dimming depth region to a deep dimming depth region can bestably made. Since control for directly driving the light-emittingdiodes 19 to 21 using a driving signal with a variable pulse width isnot used in the dimming control of the light-emitting diodes 19 to 21,flickers can be prevented from being generated in light outputs from thelight-emitting diodes compared to the dimming control based on a pulsewidth disclosed in JP-A 2003-157986 (Kokai). Also, since the need for,e.g., a switch element for the dimming control is obviated, the circuitarrangement can be simplified to reduce the number of components. As aresult, a size reduction and price reduction of the device can berealized. Furthermore, a circuit efficiency drop can be suppressed.

(Second Embodiment)

A power supply device according to the second embodiment will bedescribed below.

FIG. 7 shows the circuit arrangement of the power supply deviceaccording to the second embodiment. In the following description, thesame reference numerals denote the same parts as in FIG. 3, and adescription thereof will not be repeated.

In the circuit shown in FIG. 7, a dimming controller 41 is connected toa current detection circuit 22 and load voltage detection circuit 23. Inthe dimming controller 41, a positive input terminal of a firstoperational amplifier 42 is connected to the current detection circuit22. A negative input terminal of this first operational amplifier 42 isgrounded via a resistor 43, and is connected to its output terminal viaa resistor 44. The output terminal of the first operational amplifier 42is connected to one input terminal of a comparator 46 via a diode 45. Apositive input terminal of a second operational amplifier is connectedto the load voltage detection circuit 23. A negative input terminal ofthis second operational amplifier 47 is connected to its outputterminal, which is connected to one input, terminal of the comparator 46via a diode 48. Furthermore, a positive input terminal of the thirdoperational amplifier 50 is connected to the output terminal of thesecond operational amplifier 47. The positive input terminal of thethird operational amplifier 50 is connected to a node of a seriescircuit of resistors 51 and 52, which are connected between the outputterminal of the first operational amplifier 42 and ground, and itsnegative input terminal is grounded. Furthermore, the third operationalamplifier 50 is connected to one input terminal of the comparator 46 viaa diode 53. The comparator 46 receives a dimming signal k from a dimmingoperation member 31 at the other input terminal thereof, and inputs acomparison result of signals supplied to these input terminals to acontrol circuit 30.

In such circuit, when the dimming signal k is input from the dimmingoperation member 31, load characteristics are specified, as shown inFIG. 8, in accordance with dimming depths k1 to k5 at that time. FIG. 8shows the relationship between the load characteristics corresponding tothe dimming depths k1, k2, . . . , k5, and V-I characteristics A oflight-emitting diodes 19 to 21.

When the dimming signal k for a full lighting state, e.g., with thedimming depth k1 is input upon operation of the dimming operation member31, load characteristics corresponding to the dimming depth k1 shown inFIG. 8 are specified according to this dimming signal k. The loadcharacteristics corresponding to the dimming depth k1 are specified atan operation point a1 of the light-emitting diodes 19 to 21corresponding to an intersection between the dimming depth k1 and theV-I characteristics A of the light-emitting diodes 19 to 21. At thisoperation point a1, the output from the first operational amplifier 42according to a detection signal (current detection value I) of thecurrent detection circuit 22 is input to the comparator 46, and thelight-emitting diodes 19 to 21 are drive-controlled underconstant-current control.

Next, when the dimming signal k with the dimming depth k5 is input bythe dimming operation member 31, load characteristics corresponding tothe dimming depth k5 shown in FIG. 8 are specified according to thisdimming signal k. The load characteristics corresponding to the dimmingdepth k5 are specified at an operation point a5 of the light-emittingdiodes to 21 corresponding to an intersection between the dimming depthk5 and the V-I characteristics A of the light-emitting diodes 19 to 21.At this operation point a5, the output from the second operationalamplifier 47 according to a detection signal (load voltage V) of theload voltage detection circuit 23 is input to the comparator 46, and thelight-emitting diodes 19 to 21 are drive-controlled by the controlcircuit 30 under constant-voltage control.

Next, when the dimming signal k with the dimming depth k3 is input bythe dimming operation member 31, load characteristics corresponding tothe dimming depth k3 shown in FIG. 8 are specified according to thisdimming signal k. The load characteristics corresponding to the dimmingdepth k3 are specified at an operation point a3 of the light-emittingdiodes to 21 corresponding to an intersection between the dimming depthk3 and the V-I characteristics A of the light-emitting diodes 19 to 21.At this operation point a3, the output from the third operationalamplifier 50 according to a detection signal (current detection value I)of the current detection circuit 22 and a detection signal (load voltageV) of the load voltage detection circuit 23 is input to the comparator46, and the light-emitting diodes 19 to 21 are drive-controlled by thecontrol circuit 30 under power control.

With the aforementioned control, even when an operation point of thelight-emitting diodes 19 to 21 is located at an intermediate positionbetween the current control mode and voltage control mode, thelight-emitting diodes 19 to 21 are power-controlled based on the outputfrom the third operational amplifier 50 according to a detection signal(current detection value I) of the current detection circuit 22 and adetection signal (load voltage V) of the load voltage detection circuit23. Therefore, the control can be smoothly transited between the currentcontrol mode and voltage control mode.

(Third Embodiment)

A power supply device according to the third embodiment will bedescribed below.

The power supply device according to the third embodiment will bedescribed using FIG. 3.

In the first embodiment, a reference signal output unit 36 independentlyoutputs reference signals Vref1 and Vref2. By contrast, in this thirdembodiment, the reference signals Vref1 and Vref2 are output as a singlereference signal Vref. As shown in FIG. 9A, a first signal generator 55,which generates a reference signal Vref1 used in a region of a currentcontrol mode, and a second signal generator 56, which generates areference signal Vref2 used in a region of a voltage control mode, arearranged. The reference signals Vref1 and Vref2 of these first andsecond signal generators 55 and 56 are output via diodes 57 and 58 andtheir nodes, and the reference signal Vref shown in FIG. 9B is output tocomparators 34 and 35 as the reference signals Vref1 and Vref2 shown inFIG. 3. This reference signal Vref changes from a signal valuecorresponding to a maximum current in a full lighting state (a maximumvalue of the reference signal Vref1) to a signal value corresponding toa minimum current in case of a deepest dimming depth (a minimum value ofthe reference signal Vref1), as shown in FIG. 10A. The reference signalVref2 in the region of the voltage control mode changes from a signalvalue corresponding to a load voltage at the time of a maximum currentin a full lighting state to that corresponding to a minimum current incase of the deepest dimming depth, as shown in FIG. 10B. Even upontransition to a light-off mode in which a current ceases to flow, sincean ON voltage of light-emitting diodes on an equivalent circuit remains,the reference signal Vref2 does not become zero. According to additionof the circuit shown in FIG. 9A, the reference signal Vref in each ofthe regions of the current control mode and voltage control mode isgenerated by preferentially using a larger signal of the referencesignals Vref1 and Vref2, as shown in FIG. 9B, and this reference signalVref can be supplied to the comparators 34 and 35 as a reference signal.

By adding the circuit shown in FIG. 9A in this way, the same effects asin the first embodiment can be obtained.

(Modification)

In the above description, the reference signal Vref is generated bypreferentially using a larger signal of the reference signals Vref1 andVref2. In this case, the regions of the current control mode and voltagecontrol mode cannot often be smoothly switched. In order to smoothlyswitch the regions of the current control mode and voltage control mode,a third signal generator 59, which generates a reference signal Vref3having an intermediate gain between the reference signals Vref1 andVref2, may be arranged in addition to the first and second signalgenerators 55 and 56, as shown in FIG. 11A. The reference signals Vref1,Vref2, and Vref3 of these first to third signal generators 55, 56, and59 may be output to the comparators 34 and 35 as a reference signal Vrefshown in FIG. 11B via diodes 57, 58, and 60 and their node. In thisreference signal Vref, the reference signal Vref3, which shows anintermediate change of the gains of the reference signals Vref1 andVref2 between the regions of the current control mode and voltagecontrol mode, is included. Therefore, the control mode can be smoothlytransited between the regions of the current control mode and voltagecontrol mode.

(Fourth Embodiment)

A power supply circuit according to the fourth embodiment will bedescribed below.

FIG. 12 schematically shows the circuit arrangement of a power supplydevice according to the fourth embodiment. In the following description,the same reference numerals in FIG. 12 denote the same parts as in FIG.3, and a description thereof will not be repeated.

In the circuit shown in FIG. 12, a dimming controller 24 is connected toa current detection circuit 22 and load voltage detection circuit 23.The dimming controller 24 has a multiplier 26, adder 27, and comparator28. The multiplier 26 outputs a signal as a product of a detectionsignal (load voltage v) of the load voltage detection circuit 23 and adimming signal k from a dimming operation member 31. The adder 27generates an output a as a sum of the output from the multiplier 26 anda detection signal (current detection value I) of the current detectioncircuit 22. The comparator 28 outputs a comparison result between theoutput a of the adder 27 and a reference value (constant) 29.

To the comparator 28, a control circuit 30 which forms control meanstogether with the dimming controller 24 is connected. The controlcircuit 30 is driven by a power supply unit (not shown). The controlcircuit 30 turns on and off a switching transistor 15 by its operationto switching-drive a switching transformer 14, thereby controlling anoutput supplied from a rectifying/smoothing circuit 18 to light-emittingdiodes 19 to 21. In this control, the control circuit 30 controls anoutput to be supplied to the light-emitting diodes 19 to 21, based on anoutput from the comparator 28 in the dimming controller 24, based on avalue a obtained as a sum of the product of the detection signal (loadvoltage V) of the load voltage detection circuit 23 and the dimmingsignal k, and the detection signal (current detection value I) of thecurrent detection circuit 22, so that this value a always becomesconstant.

The dimming operation member 31 outputs the dimming signal k having adifferent dimming depth by changing, for example, a duty ratio of apulse-shaped signal. In this embodiment, the dimming operation member 31outputs the dimming signal k having dimming depths k1, k2, . . . , k7(the dimming depth k1 is shallowest, and the depth becomes deeper towardthe dimming depth k7).

In the circuit shown in FIG. 12, as the power supply device, differentload characteristics can be obtained according to the dimming depths k1,k2, . . . , k7 of the dimming signal k, as shown in FIG. 13. In thiscase, the respective load characteristics corresponding to the dimmingdepths k1, k2, . . . , k7 are radially distributed to have a constantvalue a (a position of a load impedance=0) on a current axis as acenter, and are expressed by a linear function If=a−k(Vf). In this way,a relationship given by If k(Vf)=a . . . (1), i.e., the relationship inwhich the sum of the current detection value I of the current detectioncircuit 22, and the product of the dimming signal k and load voltage valways becomes equal to the constant value a, is obtained.

The operation of the power supply circuit according to this embodimentwith the above arrangement will be described below.

Assume that the load characteristics corresponding to the dimming depthsk1, k2, . . . , k7 and V-I characteristics A of the light-emittingdiodes 19 to 21 have the relationship shown in FIG. 13.

Initially, when the dimming operation member 31 outputs the dimmingsignal k for a full lighting state, e.g., with the dimming depth k1, theload characteristics corresponding to the dimming depth k1 shown in FIG.13 are obtained according to this dimming signal k. The loadcharacteristics corresponding to the dimming depth k1 are set at anoperation point all of the light emitting diodes 19 to 21 correspondingto an intersection between the dimming depth k1 and the V-Icharacteristics A of the light-emitting diodes 19 to 21.

In this state, a switching transformer 14 is switching-driven by ON/OFFoperations of the switching transistor 15 by the control circuit 30. Inresponse to an ON operation of the switching transistor 15, a current issupplied to a primary winding 14 a of the switching transformer 14 toaccumulate an energy. In response to an OFF operation of the switchingtransistor 15, the energy accumulated on the primary winding 14 a isdischarged via a secondary winding 14 b. In this manner, a DC output isgenerated via the rectifying/smoothing circuit 18, and thelight-emitting diodes 19 to 21 are lighted on by this DC output.

In this case, the multiplier 26 calculates a product of the detectionsignal (load voltage V) of the load voltage detection circuit 23 and thedimming signal k, and the adder 27 calculates a sum of the output fromthis multiplier 26 and the detection signal (current detection value I)of the current detection circuit 22, thereby generating an output a. Thecomparator 28 then outputs a comparison result between this output a andthe reference value (constant) 29. Based on this output, the controlcircuit 30 controls an output to be supplied to the light-emittingdiodes 19 to 21, so that the output a always becomes equal to theconstant value.

With this control, at the operation point all shown in FIG. 13, a k(Vf)component which determines a in equation (1) above is nearly zero, and ais determined by only an If component (see (a0-a) in FIG. 13). In thismanner, the light-emitting diodes 19 to 21 undergo lighting controlwhile emphasizing a current control mode.

Next, when the dimming operation member 31 outputs the dimming signal kcorresponding to, e.g., the dimming depth k2, the load characteristicscorresponding to the dimming depth k2 shown in FIG. 13 are obtainedaccording to the dimming signal k at that time. Then, an intersectionbetween the load characteristics corresponding to the dimming depth k2and the V-I characteristics A of the light-emitting diodes 19 to 21 isset as an operation point a12.

In this case as well, the light-emitting diodes 19 to 21 are lighted onby ON/OFF operations of the switching transistor 15 by the controlcircuit 30 in the same manner as described above. At the operation pointa12 shown in FIG. 13, the k(Vf) component which determines a in equation(1) above is (a-a1) shown in FIG. 13, and the If component is (a0-a1)shown in FIG. 13. In this case, the If component also accounts for alarge share. Then, the light-emitting diodes 19 to 21 undergo lightingcontrol while emphasizing the current control mode.

After that, when the dimming operation member 31 outputs the dimmingsignal k for a deep dimming depth, e.g., the dimming depth k6, the loadcharacteristics corresponding to the dimming depth k6 shown in FIG. 13are obtained according to the dimming signal k at that time. Then, anintersection between the load characteristics corresponding to thedimming depth k6 and the V-I characteristics A of the light-emittingdiodes 19 to 21 is set as an operation point a16.

In this case as well, the light-emitting diodes 19 to 21 are lighted onby ON/OFF operations of the switching transistor by the control circuit30 in the same manner as described above. At the operation point a16shown in FIG. 13, the k(Vf) component which determines a in equation (1)above is (a-a6) shown in FIG. 13, and the If component is (a0-a6) shownin FIG. 13. In this state, the If component is fractional, and a isdetermined by the k(Vf) component which accounts for a large share.Then, the light-emitting diodes 19 to 21 undergo lighting control whileemphasizing a voltage control mode.

Only the operation points a11, a12, and a16 have been described. Thesame applies to other operation points a13 to 15 and a17.

Therefore, with this control, when the dimming depth is adjusted withina range of k1, k2, . . . , k7 by the dimming signal k, thelight-emitting diodes 19 to 21 undergo lighting control by setting thecurrent control mode in a shallow dimming depth region close to a fulllighting state based on the load characteristics according to thesedimming depths k1, k2, . . . , k7. As the dimming depth becomes deeper,the control mode gradually transits from the current control mode to thevoltage control mode to apply lighting control of the light-emittingdiodes 19 to 21. In this way, dimming control methods based on thecurrent control mode and voltage control mode can be smoothly transitedaccording to the dimming depths of the dimming signal, and dimmingcontrol over a broad range from a shallow dimming depth region to a deepdimming depth region can be stably attained. Since no pulse-widthcontrol is used in the dimming control, flickers can be prevented frombeing generated in light outputs from the light-emitting diodes comparedto the direct dimming control based on a pulse width disclosed JP-A2003-157986 (KOKAI). Also, since the need for, e.g., a switch elementfor the dimming control is obviated, the circuit arrangement can besimplified to reduce the number of components. As a result, a sizereduction and price reduction of the device can be realized.Furthermore, a circuit efficiency drop can be suppressed.

As is known, the V-I characteristics A of the light-emitting diodes 19to 21 have an exponentially rising region, and vary between Amax andAmin to have Acen as the center due to variations of elements and thoseof operation points caused by temperature characteristics, as shown inFIG. 14. In this case, in a shallow dimming depth region in which arelatively large current flows, the current control mode is selected, asdescribed above. Hence, a variation B12 in a region with large ΔI/ΔV canbe reduced with respect to a variation 311 in a region with small ΔI/ΔV,as shown in FIG. 15A. In a deep dimming depth region in which a smallcurrent flows, the voltage control mode is selected, as described above.Hence, a variation B22 in a region with small ΔI/ΔV can be reduced withrespect to a variation 321 in a region with large ΔI/ΔV, as shown inFIG. 15B. In this way, variations of light outputs caused by variationsof the light-emitting diodes 19 to 21 and those of operation points dueto temperature characteristics can be suppressed as much as possible.

Note that the present invention is not limited to the aforementionedembodiments, and various modifications may be made without departingfrom the scope of the invention when it is practiced. For example, inthe aforementioned embodiments, an analog circuit has been exemplified.However, a control method using a microcomputer and digital processingmay be adopted. A switching mode of the dimming depth includes acontinuous dimming mode and step-by-step dimming mode, and phase controlthat varies an effective voltage to loads by controlling a conductionperiod of a power supply voltage may be adopted. Furthermore, thedimming signal may use a dedicated signal line, and a power line Signalobtained by superposing the dimming signal on a power supply line may beused.

According to the embodiment, by only selecting the reference signalaccording to the dimming depth of the dimming signal, the currentcontrol mode and voltage control mode can be easily switched.

According to the embodiment, smooth control transition is allowedbetween the current control mode and voltage control mode.

According to the embodiment, the dimming control methods in which thecurrent control mode and voltage control mode are emphasized can besmoothly switched by only changing the dimming depth, and dimmingcontrol over a broad range from a shallow dimming depth region to a deepdimming depth region can be stably attained.

According to the embodiment, variations of light outputs caused byvariations of semiconductor light-emitting elements and those ofoperation points due to temperature characteristics can be suppressed.

According to the embodiment, a lighting equipment which can implementstable dimming control can be provided, and a power supply device andlighting equipment which can implement stable dimming control can beprovided.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intended,to cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A power supply device comprising: a semiconductorlight-emitting element; a current detection circuit configured to detecta current supplied to the semiconductor light-emitting element andoutput a current detection signal; a voltage detection circuitconfigured to detect a voltage applied to the semiconductorlight-emitting element and output a voltage detection signal; a dimmingcontroller configured to receive a dimming signal indicating one of aplurality of dimming depths, output a first reference signal having alevel determined in accordance with the dimming depth of the dimmingsignal, and output a second reference signal having a level determinedin accordance with the dimming depth of the dimming signal, and acontrol circuit configured to: when the first reference signal is higherthan the second reference signal, receive a comparison result betweenthe first reference signal and the current detection signal and set acurrent control mode to control electrical power supplied to thesemiconductor light-emitting element, and when the first referencesignal is lower than the second reference signal, receive a comparisonresult between the second reference signal and the voltage detectionsignal and set a voltage control mode to control electrical powersupplied to the semiconductor light-emitting element, wherein: when thecontrol circuit sets the current control mode, the control circuitdetermines a target current based on the dimming depth to control adirect electrical current, which is continuously supplied to thesemiconductor light-emitting element, in such a manner that the directelectrical current is kept at the target current, and when the controlcircuit sets the voltage control mode, the control circuit determines atarget voltage to control a load voltage applied to the semiconductorlight-emitting element in such a manner that the load voltage is kept atthe target voltage.
 2. The power supply device according to claim 1,wherein: the first reference signal is utilized to control the directelectrical current at the target current; and the second referencesignal is utilized to control the load voltage at the target voltage. 3.The power supply device according to claim 2, wherein the controlcircuit includes a power control module configured to control theelectrical power supplied to the semiconductor light-emitting elementbased on the current supplied to the semiconductor light-emittingelement and the load voltage applied to the semiconductor light-emittingelement in an intermediate region which is defined between aconstant-current control region in which the current control mode isperformed and a constant-voltage control region in which the voltagecontrol mode is performed.
 4. A lighting equipment comprising: a powersupply device according to claim 3, and an equipment body having thepower supply device.
 5. A lighting equipment comprising: a power supplydevice according to claim 2, and an equipment body having the powersupply device.
 6. The power supply device according to claim 1, whereinthe control circuit includes a power control module configured tocontrol the electrical power supplied to the semiconductorlight-emitting element based on the current supplied to thesemiconductor light-emitting element and the load voltage applied to thesemiconductor light-emitting element in an intermediate region which isdefined between a constant-current control region in which the currentcontrol mode is performed and a constant-voltage control region in whichthe voltage control mode is performed.
 7. A lighting equipmentcomprising: a power supply device according to claim 6, and an equipmentbody having the power supply device.
 8. A lighting equipment comprising:a power supply device according to claim 1, and an equipment body havingthe power supply device.